Refractive index system monitor and control for immersion lithography

ABSTRACT

A system and/or method are disclosed for measuring and/or controlling refractive index (n) and/or lithographic constant (k) of an immersion medium utilized in connection with immersion lithography. A known grating structure is built upon a substrate. A refractive index monitoring component facilitates measuring and/or controlling the immersion medium by utilizing detected light scattered from the known grating structure.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.10/645,363, filed on Aug. 21, 2003, now U.S. Pat. No. 6,844,206 entitledREFRACTIVE INDEX SYSTEM MONITOR AND CONTROL FOR IMMERSION LITHOGRAPHY,hereby incorporated herein by reference.

TECHNICAL FIELD

The present invention generally relates to semiconductor processing and,more particularly, to a system and method for monitoring and/orcontrolling refractive index of an immersion medium utilized inconnection with immersion lithography.

BACKGROUND OF THE INVENTION

In the semiconductor industry, there is a continuing trend toward higherdevice densities. To achieve these high device densities there havebeen, and continue to be, efforts toward scaling down device dimensions(e.g., at sub-micron levels) on semiconductor wafers. In order toaccomplish such densities, smaller feature sizes and more precisefeature shapes are required. This may include width and spacing ofinterconnecting lines, spacing and diameter of contact holes, andsurface geometry, such as corners and edges, of various features. Thedimensions of and between such small features can be referred to ascritical dimensions (CDs). Reducing CDs and reproducing more accurateCDs facilitates achieving higher device densities.

High resolution lithographic processes are used to achieve smallfeatures. In general, lithography refers to processes for patterntransfer between various media. In lithography for integrated circuitfabrication, a silicon slice, the wafer, is coated uniformly with aradiation-sensitive film, the photoresist. The film is selectivelyexposed with radiation (e.g., optical light, x-ray, electron beam, . . .) through an intervening master template (e.g., mask, reticle, . . . )forming a particular pattern (e.g., patterned resist). Dependent uponcoating type, exposed areas of the coating become either more or lesssoluble than unexposed areas in a particular solvent developer. Moresoluble areas are removed with the developer in a developing step, whileless soluble areas remain on the silicon wafer to form a patternedcoating. The pattern corresponds to either the image of the mask or itsnegative. The patterned resist is used in further processing of thesilicon wafer.

Efforts to reduce CDs have included implementing various techniques inconnection with the lithographic process, such as reducing exposureradiation wavelength (e.g., from 436 nm mercury g-line to 365 nm i-lineto 248 nm DUV to 193 nm excimer laser), improving optical design,utilizing metrology techniques (e.g., scatterometry, scanning electronmicroscope (SEM)), etc. Immersion lithography facilitates furtherreduction of CDs.

In immersion lithography, the gap between a substrate (e.g., wafer) anda final optical component (e.g., lens) is filled with an immersionmedium, which has a higher refractive index than air. Refractive indexis defined as a ratio of speed of light in a vacuum to speed of light ina particular medium. Utilizing an immersion medium with a refractiveindex greater than that of air, which approximately equals 1, canincrease numerical aperture, which is defined as a lens's ability togather diffracted light and resolve fine details onto a wafer.Furthermore, utilization of an immersion medium can decrease aneffective wavelength of an exposure radiation propagating within theimmersion medium without changing exposure radiation, lasers, lensmaterials, etc.

Currently, immersion lithography is limited by an inability to monitorand control immersion medium properties such as, for example, refractiveindex (n) and lithography constant (k). Conditions that can impact theseproperties include, for example, temperature, pressure, formation ofmicrobubbles, chemical contamination of fluid, thermal and mechanicalchanges, etc. These conditions can impact efficiency of immersionlithography systems and can elevate costs for expensive immersionmediums. Thus, there exists a need in the art for systems and methodsthat can monitor and/or control immersion medium properties.

SUMMARY OF THE INVENTION

The following presents a simplified summary of the invention in order toprovide a basic understanding of some aspects of the invention. Thissummary is not an extensive overview of the invention. It is intended toneither identify key or critical elements of the invention nor delineatethe scope of the invention. Its purpose is merely to present someconcepts of the invention in a simplified form as a prelude to the moredetailed description that is presented later.

The present invention provides a system and method for monitoring and/orcontrolling refractive index and lithographic constant values of animmersion medium. An immersion medium in accordance with the inventioncan occupy a gap between a final optical component and a substrate, andhave a refractive index greater than air. Furthermore, according toanother aspect of the present invention, the immersion medium can be100% transparent to an exposure wavelength. Additionally, the immersionmedium can be water, supercritical fluid in gaseous phase, ozone vapor,etc. A known grating structure is constructed upon the substrate—arefractive index monitoring component emits an incident beam thatinteracts with the immersion medium and the known grating structure uponthe substrate to produce a reflected and/or diffracted beam(s). Thereflected and/or diffracted beam is analyzed to derive characteristicsof the immersion medium such as, for example refractive index (n) andlithographic constant (k) values. According to an aspect of the presentinvention, the refractive index monitoring component can effectuatechanges in refractive index and/or lithographic constant of theimmersion medium by means of varying temperature, pressure, flushing theimmersion medium, etc.

Another aspect of the present invention provides a system for measuringand/or controlling refractive index of an immersion medium. The systemincludes a refractive index monitoring component comprising a measuringcomponent and a control component. The measuring component utilizes alight source which emits light that interacts with an immersion mediumand a known grating structure, and a detector which can receivereflected and/or diffracted light. Refractive index and lithographicconstant values can be derived employing information related to theemitted and received beams.

Another aspect of the present invention provides a method for measuringand/or controlling refractive index of an immersion medium. A substratewith a known grating structure built upon it is at least partiallyimmersed in an immersion medium. An incident light beam is emitted intothe immersion medium and onto the substrate. As a result,characteristics of the immersion medium such as, for example, refractiveindex and lithographic constant, can be derived. Furthermore, changes tosuch characteristics of the immersion medium can be effectuated by meansof changing temperature, pressure, etc. of the immersion medium.

To the accomplishment of the foregoing and related ends, certainillustrative aspects of the invention are described herein in connectionwith the following description and the annexed drawings. These aspectsare indicative, however, of but a few of the various ways in which theprinciples of the invention may be employed and the present invention isintended to include all such aspects and their equivalents. Otheradvantages and novel features of the invention will become apparent fromthe following detailed description of the invention when considered inconjunction with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration of a system for measuring and/or controllingrefractive index of an immersion medium in accordance with an aspect ofthe present invention.

FIG. 2 is an illustration of a system for measuring and/or controllingrefractive index of an immersion medium in accordance with an aspect ofthe present invention.

FIG. 3 is an illustration of a system for measuring and/or controllingrefractive index of an immersion medium in accordance with an aspect ofthe present invention.

FIG. 4 is a schematic block diagram of an exemplary refractive indexmonitoring system in accordance with an aspect of the present invention.

FIG. 5 is a schematic block diagram of an exemplary refractive indexmonitoring system in accordance with an aspect of the present invention.

FIG. 6 is an illustration of a substrate in accordance with an aspect ofthe present invention.

FIG. 7 is a flow diagram of a refractive index monitoring and/ormeasuring methodology in accordance with an aspect of the presentinvention.

FIG. 8 is a flow diagram of a refractive index monitoring and/ormeasuring methodology in accordance with an aspect of the presentinvention.

FIG. 9 illustrates a perspective view of a grid-mapped wafer accordingto one or more aspects of the present invention.

FIG. 10 illustrates plots of measurements taken at grid-mapped locationson a wafer in accordance with one or more aspects of the presentinvention.

FIG. 11 illustrates a table containing entries corresponding tomeasurements taken at respective grid-mapped locations on a wafer inaccordance with one or more aspects of the present invention.

FIG. 12 is a simplified perspective view of an incident light reflectingoff a surface in accordance with one or more aspects of the presentinvention.

FIG. 13 is a simplified perspective view of an incident light reflectingoff a surface in accordance with one or more aspects of the presentinvention.

FIG. 14 is an illustration of a complex reflected and refracted lightproduced when an incident light is directed onto a surface in accordancewith one or more aspects of the present invention.

FIG. 15 is an illustration of a complex reflected and refracted lightproduced when an incident light is directed onto a surface in accordancewith one or more aspects of the present invention.

FIG. 16 is an illustration of a complex reflected and refracted lightproduced when an incident light is directed onto a surface in accordancewith one or more aspects of the present invention.

FIG. 17 is an illustration of phase and/or intensity signals recordedfrom a complex reflected and refracted light produced when an incidentlight is directed onto a surface in accordance with one or more aspectsof the present invention.

FIG. 18 is an illustration of an exemplary computing system and/orenvironment in connection with facilitating employment of the subjectinvention.

DETAILED DESCIPTION OF THE INVENTION

The present invention is now described with reference to the drawings,wherein like reference numerals are used to refer to like elementsthroughout. In the following description, for purposes of explanation,numerous specific details are set forth in order to provide a thoroughunderstanding of the present invention. It may be evident, however, toone skilled in the art that one or more aspects of the present inventionmay be practiced with a lesser degree of these specific details. Inother instances, known structures and devices may be shown in blockdiagram form in order to facilitate describing one or more aspects ofthe present invention.

As used in this application, the term “component” is intended to referto a computer-related entity, either hardware, a combination of hardwareand software, software, or software in execution. For example, acomponent may be, but is not limited to being a process running on aprocessor, a processor, an object, an executable, a thread of execution,a program, a set of co-operating computers and/or processes and acomputer.

It is to be appreciated that various aspects of the present inventioncan employ technologies associated with facilitating unconstrainedoptimization and/or minimization of error costs. Thus, non-lineartraining systems/methodologies (e.g., back propagation, Bayesian, fuzzysets, non-linear regression, or other neural networking paradigmsincluding mixture of experts, cerebella model arithmetic computer(CMACS), radial basis functions, directed search networks, and functionline networks) can be employed. The invention can employ variousinference schemes and/or techniques in connection with statedetermination, inference and/or prediction. As used herein, the term“inference” refers generally to the process of reasoning about orinferring states of the system, environment, and/or user from a set ofobservations as captured via events and/or data. Inference can beemployed to identify a specific context or action, or can generate aprobability distribution over states, for example. The inference can beprobabilistic—that is, the computation of a probability distributionover states of interest based on a consideration of data and events.Inference can also refer to techniques employed for composinghigher-level events from a set of events and/or data. Such inferenceresults in the construction of new events or actions from a set ofobserved events and/or stored event data, whether or not the events arecorrelated in close temporal proximity, and whether the events and datacome from one or several event and data sources. Various classificationschemes and/or systems (e.g., support vector machines, neural networks,expert systems, Bayesian belief networks, fuzzy logic, data fusionengines . . . ) can be employed in connection with performing automaticand/or inferred action in connection with the subject invention.

FIG. 1 illustrates a block diagram of a system 100 for measuring and/orcontrolling refractive index of an immersion medium 110 in accordancewith an aspect of the present invention. The immersion medium 110occupies the gap between a substrate 120 (e.g., wafer) and a finaloptical component 130 (e.g., lens). Characteristics of the immersionmedium 110 can include low optical absorption at the exposure radiationwavelength, compatibility with resist and lens material, uniformity ofproperties throughout the immersion medium 110, non-contaminating, etc.According to an aspect of the present invention, the immersion medium110 can be, for example, water, oil (e.g., perfluorinated polyethers(PFPE) including PFPE-K, PFPE-Y, PFPE-D, PFPE-M, PFPE-Z), etc.

The immersion medium 110 utilized in connection with the presentinvention has a refractive index (n) greater than the refractive indexof air. Refractive index is a ratio of speed of light in a vacuum tospeed of light in a particular medium and varies dependent uponradiation wavelength. For example, the refractive index corresponding to193 nm radiation is approximately 1.4 for water and approximately 1 forair. Furthermore, it is desirable to have a refractive index greaterthan 1 provided that the immersion medium 110 is 100% transparent to theexposure radiation wavelength. However, the invention is not intended tobe limited to a 100% transparent immersion medium 110 with refractiveindex greater than 1, as various other transparencies are contemplatedand intended to fall within the scope of the hereto appended claims.

The immersion medium 110 interacts with a refractive index monitoringcomponent 140 that can monitor and/or control the refractive index ofthe immersion medium 110. The refractive index monitoring component 140can derive the refractive index (n) and/or lithographic constant (k) ofthe immersion medium 110. Additionally, the refractive index monitoringcomponent 140 can control refractive index and/or lithographic constantvalues of the immersion medium 110. For example, control of refractiveindex and/or lithographic constant of the immersion medium 110 can befacilitated by means of modifying temperature, altering pressure,flushing the immersion medium from the system and adding new liquid,etc.

Refractive index and/or lithographic constant values can be derived fromlight scattering of a known grating structure which can be built uponthe substrate 120 (e.g., wafer, wafer stage, reticle, . . . ). Thesubstrate 120 thus can be at least partially immersed within theimmersion medium 110 and subjected to radiation, which is detected andanalyzed to yield values for refractive index and/or lithographicconstant. It is to be appreciated that the refractive index monitoringcomponent 140 can be, for example, a scatterometry component. Thepresent invention contemplates any suitable refractive index monitoringcomponent 140 and/or system, and such components and/or systems areintended to fall within the scope of the hereto-appended claims.

Tunable values for refractive index and/or lithographic constantfacilitate additional control over resolution of the photolithographicprocess. The refractive index monitoring component 140 can facilitatechange of the characteristics (e.g., refractive index, lithographicconstant, . . . ) of the immersion medium 110, thus resulting in controlof resolution of the photolithographic process. In particular, theresolution can be defined as:resolution=kλ/NAwhere k is a lithographic constant, A is an exposure radiationwavelength, and NA is a numerical aperture. Furthermore, the numericalaperture can be derived as follows:NA=n sin αwhere n is a refractive index and 2 α is an angle of acceptance of alens. Thus, resolution can be increased by increasing refractive indexand/or decreasing lithographic constant.

The immersion medium 110 can be a supercritical fluid in gaseous phaseor ozone vapor—a supercritical fluid is a fluid that is at a temperatureand pressure above a particular medium's critical temperature andcritical pressure. While in a supercritical state, pressure can beapplied to the medium to effectuate a change in the refractive indexand/or lithographic constant of the immersion medium 110.

It is further to be appreciated that information gathered by therefractive index monitoring component 140 can be utilized for generatingfeedback and/or feed-forward data that can facilitate achievingincreased resolution. The system 100 for monitoring and/or controllingrefractive indices can additionally employ such data to controlcomponents and/or operating parameters associated therewith. Forinstance, feedback/feed-forward information can be generated inconnection with the immersion medium to maintain, increase, or decreasetemperature and/or pressure of the immersion medium.

FIG. 2 illustrates another example of a system 200 for measuring and/orcontrolling refractive index of an immersion medium 210 in accordancewith an aspect of the present invention. The system comprises animmersion medium 210 which can possess a refractive index greater than1, which facilitates lowering effective wavelength of exposure radiationpropagating within the immersion medium 210. For example, the immersionmedium 210 can be water, supercritical fluid in gaseous phase, ozonevapor, etc. An optical component 230, such as a lens, can be employed toemit light to a substrate 220 (e.g., a wafer) via the immersion medium210, thereby effectuating a particular semiconductor fabrication processsuch as wafer etching. The immersion medium 210 is further operativelycoupled to a refractive index monitoring component 240. The refractiveindex monitoring component 240 comprises a measurement component 250 anda control component 260.

The measurement component 250 can determine refractive index (n) and/orlithography constant (k) values of the immersion medium 210. Themeasurement component 250, for example, can include a light source thatemits an incident beam onto a substrate 220 (e.g., wafer, wafer stage,reticle, . . . ), which has a known grating structure. When the beam isemitted, it interacts with the substrate 220 and the immersion medium210 and is diffracted and/or reflected. The measurement component 250can also include a detection system, such as a spectrometer, fordetecting the reflected and/or diffracted beam from the substrate 220and immersion medium 210. Characteristics of the immersion medium 210are determined based on properties of the reflected and/or diffractedbeam and the known grating structure. Those skilled in the art willunderstand and appreciate various other non-destructive opticalmeasurement techniques that could be utilized.

The control component 260 can be operatively coupled to the immersionmedium 210 and to the measurement component 250. The control component260 is programmed and/or configured to control operation of themeasurement component 250 and characteristics of the immersion medium210 in accordance with an aspect of the present invention.

According to one particular aspect of the present invention, the controlsystem 260 can control the measurement component 250 so that theincident beam selectively interrogates an immersion medium 210 and aknown grating structure on a substrate 220. For example, the incidentbeam can be emitted to selectively interrogate the immersion medium 210when the substrate 220 with the known grating structure is at leastpartially within the immersion medium 210.

Alternatively or additionally, the control component 260 can controlimmersion medium 210 characteristics such as refractive index and/orlithographic constant. The control component 260 can effectuate a changein characteristics of the immersion medium 210 by varying temperature,pressure, flushing immersion medium, etc. The control system 260 canutilize measured characteristic data of the immersion medium 210 toappropriately control changes of characteristics of the immersion medium210 to facilitate optimization of the immersion lithography process.

FIG. 3 illustrates another example of a system 300 for measuring and/orcontrolling refractive index of an immersion medium 310 in accordancewith an aspect of the present invention. The refractive index measuringand/or controlling system 300 can employ various inference schemesand/or techniques in connection with measuring and controllingrefractive index of the immersion medium 310. As used herein, the term“inference” refers generally to the process of reasoning about orinferring states of the system, environment, and/or user from a set ofobservations as captured via events and/or data. Inference can beemployed to identify a specific context or action, or can generate aprobability distribution over states, for example. The inference can beprobabilistic—that is, the computation of a probability distributionover states of interest based on a consideration of data and events.Inference can also refer to techniques employed for composinghigher-level events from a set of events and/or data. Such inferenceresults in the construction of new events or actions from a set ofobserved events and/or stored event data, whether or not the events arecorrelated in close temporal proximity, and whether the events and datacome from one or several event and data sources. Various classificationschemes and/or systems (e.g., support vector machines, neural networks,expert systems, Bayesian belief networks, fuzzy logic, data fusionengines . . . ) can be employed in connection with performing automaticand/or inferred action in connection with the subject invention.

Still referring to FIG. 3, a refractive index measuring and/orcontrolling system 300 comprises an immersion medium 310 (e.g., water,oil, supercritical fluid in gaseous state, ozone vapor, . . . ) that canpossess a refractive index greater than 1, which facilitates loweringeffective wavelength of exposure radiation. An optical component 330,such as a lens, can be employed to emit light to a substrate 320 (e.g.,a wafer) via the immersion medium 310, thereby effectuating a particularsemiconductor fabrication process such as wafer etching. A refractiveindex monitoring component 340 is operatively coupled to the immersionmedium 310, comprising a measurement component 350 and a controlcomponent 360. The refractive index monitoring component 340 can beoperatively coupled to an artificial intelligence (AI) component 370that is capable of making inferences regarding system operation and adata store 380 that can store data corresponding to known gratingstructures on substrates 320, prior immersion medium 310characteristics, previous changes to the immersion medium 310 by meansof change in temperature, pressure, etc. Additionally, the AI component370 can be operatively coupled to the data store 380. According to anaspect of the present invention, the AI component 370 can determineoptimal changes to the immersion medium 310 which can be effectuated bythe control component 360. Furthermore, the AI component 370 and/orrefractive index monitoring component 340 can store and retrieve datafrom the data store 380 corresponding to the immersion medium 310 suchas, for example, refractive index values, lithographic constant values,temperatures, pressures, changes implemented to immersion medium, etc.These examples are given by way of illustration only and are not in anyway intended to limit the scope of the present invention or the numberof, or manner in which the AI component 370 makes inferences.

FIG. 4 illustrates another example of a system 400 for measuring and/orcontrolling refractive index of an immersion medium 402 in accordancewith an aspect of the present invention. In this example, a substrate404 (e.g., wafer, wafer stage, reticle, . . . ) with a known gratingstructure is at least partially within an immersion medium 402 (e.g.,water, supercritical fluid in gaseous state, ozone vapor, . . . ).Additionally, a final optical component, such as a lens 406, is also atleast partially within the immersion medium 402. The system can alsoutilize a measuring system 408 for measuring features of the immersionmedium 402 in accordance with an aspect of the present invention. By wayof illustration, the measuring system 408 is a non-destructivemeasurement tool that includes a source of light 410, such as one ormore optical emitters, for emitting an incident light beam 412 towardthe substrate 404 at an incident angle θ relative to a normal referenceline. The light source 410 can be a frequency stabilized laser; howeverit will be appreciated by one skilled in the alt that any laser or othersuitable light source (e.g., laser diode, or helium neon (HeNe) gaslaser, halogen lamp, etc.) can be utilized in connection with thepresent invention.

At least a portion of the incident beam 412 is reflected and/ordiffracted as a reflected beam 414. One or more optical detectors 416receive(s) the reflected and/or diffracted beam 414. The detector(s) 416analyze characteristics of the reflected beam 414 and can be operativeto discern optical properties of the beam. As described below, theoptical properties of the beam describe optical characteristics of theknown grating structure built on the substrate 404 and the immersionmedium 402, which facilitates deriving characteristics of the immersionmedium 402. The immersion medium 402 characteristics, for example, caninclude refractive index and lithographic constant.

By way of example, the detector 416 can include a spectrometer or anyinstrument capable of providing spectrally-resolved informationconcerning the reflected beam 414. The portion of the reflected beam 414that enters the detector 416 for analysis is determined by such portionand its associated diffraction characteristics, the special extent ofthe reflected beam 414, properties of the detector 416, and anyassociated optical elements that might be used in conjunction with thedetector 416.

The detector 416 collects light reflected and/or passed through one ormore gratings and/or features built upon the substrate 404 and theimmersion medium 402. The measurement system 408 can extract informationregarding the characteristics of the immersion medium 402 by comparingphase and/or intensity of the incident beam 412 with phase and/orintensity signals of a complex reflected and/or diffracted lightassociated with the reflected beam 414. The substrate 404 has a knowngrating structure constructed upon it which allows for derivingcharacteristics of the immersion medium 402. The intensity and/or thephase of the reflected and/or diffracted light changes based onproperties of the immersion medium 402.

The system can further comprise a control component 418. According toone aspect of the present invention, the control component 418 can beoperatively coupled to the lens 406, measurement system 408 and theimmersion medium 402. The control component 418 can facilitate operationof the measurement system 408. Additionally, the control component 418can effectuate changes in temperature, pressure, etc. of the immersionmedium 402 to change refractive index and/or lithographic constantvalues based at least in part upon measurements obtained by themeasuring system 408. Moreover, the control component 418 can furtherutilize such measurements to control operation of optical componentssuch as, for example, the lens 406. Control of the lens 406 facilitatescontrol over an exposure germane to a photolithographic process.

According to this aspect, a control component 418 further comprises aprocessor 420 and memory 422. It is to be understood that the processor420 can be a processor dedicated to determining refractive index and/orlithographic constant values of the immersion medium 402, a processorused to control the immersion medium 402 thereby tuning the refractiveindex and/or lithographic constant values, or, alternatively, aprocessor that is both used to determine refractive index and/orlithographic constant values and to control tuning of the values.

The memory 422 stores program code executed by the processor 420 forcarrying out operating functions of the system. The memory 422 alsoserves as a storage medium for temporarily storing information, such asrefractive index, lithographic constant, temperature, pressure, etc.that can be employed in carrying out the present invention. The memory422 can be either volatile memory or nonvolatile memory, or can compriseboth volatile and nonvolatile memory. By way of illustration, and notlimitation, nonvolatile memory can comprise read only memory (ROM),programmable ROM (PROM), electrically programmable ROM (EPROM),electrically erasable ROM (EEPROM), or flash memory. Volatile memory cancomprise random access memory (RAM), which acts as external cachememory. By way of illustration and not limitation, RAM is available inmany forms such as synchronous RAM (SRAM), dynamic RAM (DRAM),synchronous DRAM (SDRAM), double data rate SDRAM (DDR SDRAM), enhancedSDRAM (ESDRAM), Synchlink DRAM (SLDRAM), and direct Rambus RAM (DRRAM).The memory of the present systems and methods is intended to comprise,without being limited to, these and any other suitable types of memory.

FIG. 5 illustrates an arrangement that is similar in function to FIG. 4,and which accommodates substrates that are partially or fullytransparent. The substrate 502 is at least partially within theimmersion medium 504. In this example, a light source 506 provides anincident beam 508, at least a portion of which is transmitted throughthe substrate 502 and immersion medium 504. A diffracted and/orrefracted beam 510 exits a side of the substrate 502 opposite to that ofthe incident beam 508. At least a portion of the diffracted beam entersa detector 512, such as a spectrometer, which can be processed usingknown scatterometry techniques as described herein.

By way of example, a control component 514, which is operatively coupledto the light source 506, the detector 512, and the immersion medium 504,is programmed and/or configured to control operation and/orcharacteristics of such components. In particular, the control component514 includes a measurement component 516 programmed and/or configured tocontrol operation of the light source 506 and the detector 512. Asindicated below, the measurement component 516 also is programmed todetermine characteristics of the immersion medium 504 based on opticaldata obtained by the detector 512 (e.g., spectrometer) based on the beam510 and a known grating structure upon the substrate 502. For example,the measurement component 516 can be utilized to obtain refractive indexand/or lithographic constant values of the immersion medium 504.

The control component 514 also is programmed and/or configured tocontrol characteristics of the immersion medium 504. The controlcomponent 514 can control temperature, pressure, exchange immersionmedium, etc. to effectuate a change in characteristics of the immersionmedium 504. Such characteristic changes can include, for example,varying refractive index, lithographic constant, etc.

Alternatively or additionally, the control component 514 can controlcharacteristics of the immersion medium 504 based at least in part onmeasured characteristics of the immersion medium 504 obtained via themeasurement component 516. In this way, operation of the measuringcomponent 516 and the immersion medium 504 can be synchronized tofacilitate tunable refractive index and lithographic constant values ofthe immersion medium 504, which thus accommodate increased resolution.

Referring to FIG. 6, an example of a substrate 600 (e.g., wafer, waferstage) such as can be utilized in connection with immersion lithographyis illustrated for use in accordance with an aspect of the presentinvention. Gratings 602 are located near a central region of thesubstrate 600 to facilitate inspection and/or measurement of animmersion medium. The gratings 602 can be located between productionregions of the substrate 604 so as to maximize real estate associatedwith the substrate 600 being manufactured. Alternatively, the gratings602 can be located upon a wafer stage and/or a wafer not subject toproduction. The particular grating 602 illustrated in FIG. 6 is a seriesof elongated parallel marks, which can be implemented as raised portionsin the substrate or as troughs, such as etched into the substrate 600.It is to be appreciated that more complex (e.g., nonlinear) gratingpatterns and/or substrate features (e.g., lines, connectors, . . . )also could be used in accordance with an aspect of the presentinvention. A known grating structure can be employed in connection withthe present invention and thus can facilitate deriving characteristicsof an immersion medium by utilizing measurement techniques associatedwith the combination of substrate 600 and immersion medium.

In view of the exemplary systems shown and described above,methodologies 700 and 800, which may be implemented in accordance withthe present invention, will be better appreciated with reference to theflow diagrams of FIG. 7 and FIG. 8. While, for purposes of simplicity ofexplanation, the methodologies 700 and 800 are shown and described as aseries of function blocks, it is to be understood and appreciated thatthe present invention is not limited by the order of the blocks, as someblocks may, in accordance with the present invention, occur in differentorders and/or concurrently with other blocks from that shown anddescribed herein. Moreover, not all illustrated blocks may be requiredto implement a methodology in accordance with the present invention. Itis to be appreciated that the various blocks may be implemented viasoftware, hardware a combination thereof or any suitable means (e.g.,device, system, process, component) for carrying out the functionalityassociated with the blocks. It is also to be appreciated that the blocksare merely to illustrate certain aspects of the present invention in asimplified form and that these aspects may be illustrated via a lesserand/or greater number of blocks.

Turning to FIG. 7, the methodology 700 initializes operatingcharacteristics to their starting values at 710. This can include, forexample, placing a substrate with a known grating structure at leastpartially into the immersion medium and/or setting initial opticalparameters of an incident beam for measuring topographicalcharacteristics of the substrate in accordance with an aspect of thepresent invention.

At 720, an incident beam is emitted. The incident beam, for example, isemitted so as to interrogate a known grating structure upon a substrateand/or the immersion medium. The known grating structure can be, forexample, substantially parallel lines built upon a substrate. At 730, adiffracted and/or reflected beam produced from the incident beaminteracting with the substrate and immersion medium is detected. Thereflected and/or diffracted beam, for example, is collected by aspectrometer or other optical detection device capable of detectingproperties of the reflected and/or diffracted beam. The reflected and/ordiffracted beam contains useful, quantifiable information indicative ofoptical characteristics of the substrate and immersion medium.

At 740, optical properties and characteristics of the reflected and/ordiffracted beam are determined. The optical properties, for example, caninclude wavelength(s) and intensity of light, refractive indices,polarization state, etc. of the reflected and/or diffracted beam. Theoptical properties can be employed with known grating structures buildupon a substrate to derive characteristics of the immersion medium(e.g., refractive index, lithographic constant, . . . ).

From 740, the process returns to 720 where the foregoing methodology isrepeated, such as for a time while the substrate with the known gratingstructure is within the immersion medium. The determined characteristicsof the immersion medium can, in turn, be utilized to adjust temperature,pressure, etc. of the immersion medium, thus adjusting properties of theimmersion medium.

FIG. 8 is a flow diagram illustrating another methodology 800 forcarrying out the present invention. At 810 the operating characteristicsare initialized to their starting values. This can include, for example,placing a substrate with a known grating structure at least partiallyinto the immersion medium and/or setting initial optical parameters ofan incident beam for measuring topographical characteristics of thesubstrate in accordance with an aspect of the present invention.

At 820, an incident beam is emitted. The incident beam, for example, isemitted as to interrogate an immersion medium and/or a substrate (e.g.,wafer, wafer stage, . . . ). A known grating structure built upon thesubstrate. The incident beam is reflected and/or diffracted to produce abeam having characteristics indicative of substrate and/or immersionmedium properties illuminated by the incident beam.

At 830, the reflected and/or diffracted beam is detected, such as usinga spectrometer, although other optical detection techniques capable ofdetecting the reflected and/or diffracted beam could be used. At 840,optical characteristics of the reflected and/or diffracted beam, such asintensity of one or more wavelengths of the detected light, phasecharacteristics, refractive indices, polarization state, etc., aredetermined. The optical characteristics can be employed to derive anindication of immersion medium parameters, such as refractive index andlithographic constant.

At 850, a determination is made as to whether the immersion mediumcharacteristics are within an expected range. If the immersion mediumcharacteristics are within an expected range of parameters, the processreturns to 820 and the foregoing methodology is repeated. If thedetermination 850 is negative, indicating that immersion mediumcharacteristics are outside the expected range, the process proceeds to860. At 860, immersion medium characteristics are adjusted by means oftemperature change, pressure change, flushing immersion medium, etc.

From 860, the present iteration ends and the process returns to 820, inwhich the methodology continues, as described above such as for aduration commensurate with the associated fabrication process. As aresult, the present invention facilitates controlling refractive indexand lithographic constant of an immersion medium at a fixed exposurewavelength, and thus increases resolution.

Turning now to FIGS. 9–11, in accordance with one or more aspects of thepresent invention, a wafer 902 situated on a stage 904 can be logicallypartitioned into grid blocks. Each grid block (XY) of the grid patterncorresponds to a particular portion of the wafer 902, and each gridblock has a known grating structure associated with that grid block.Each portion is monitored individually for signatures generated by theknown grating structure and a portion of an immersion medium.

In FIG. 10, one or more portions of the immersion medium and the knowngrating structures in respective portions of the wafer 902 (X₁Y₁ . . .X₁₂, Y₁₂) are being monitored for signatures using reflective and/orpassed through light, a signature system and a processor. It is to beappreciated that although FIG. 10 illustrates the wafer 902 being mapped(partitioned) into 144 grid block portions, the wafer 902 may be mappedwith any suitable number of portions and any suitable number of gratingsmay be employed. Given the set of recorded signatures, a processor candetermine that an undesirable immersion medium characteristic (e.g.,refractive index, lithographic constant) exists. Similarly, a processormay generate feed forward information which can facilitate maintaining,terminating, and/or adjusting conditions associated with the immersionmedium such as, for example temperature, pressure, etc.

FIG. 11 illustrates a table of expected and unexpected signatures. Itcan be seen that all the signatures are expected except a signature forgrid X₇Y₆. The set of depicted signatures can be analyzed collectivelyas a master signature and/or can be analyzed in subsets to evaluate, forexample, refractive index and lithographic constant values. The analysisof the signatures can be employed to control characteristics of theimmersion medium such as, for example refractive index and/orlithographic constant. Furthermore, temperature, pressure, etc. can bemonitored and/or controlled in connection with the grid blocks.

Scatterometry is a technique for extracting information about a surfaceupon which an incident light has been directed. Information concerningproperties including, but not limited to, dishing, erosion, profile,chemical composition, thickness of thin films and critical dimensions offeatures present on a surface such as a wafer can be extracted.Furthermore, information about an immersion medium such as refractiveindex and lithographic constant can be extracted by utilizingscatterometry techniques. The information can be extracted by comparingthe phase and/or intensity of the light directed onto the surface withphase and/or intensity signals of a complex reflected and/or diffractedlight resulting from the incident light reflecting from and/ordiffracting through the surface upon which the incident light wasdirected. The intensity and/or the phase of the reflected and/ordiffracted light will change based on properties of the surface uponwhich the light is directed and the immersion medium which the lighttravels through. Such properties include, but are not limited to, thechemical properties of the surface, the planarity of the surface,features on the surface, voids in the surface, and the number, type oflayers beneath the surface, refractive index of the medium, lithographicconstant of the medium.

Different combinations of the above-mentioned properties will havedifferent effects on the phase and/or intensity of the incident lightresulting in substantially unique intensity/phase signatures in thecomplex reflected and/or diffracted light. Thus, by examining a signal(signature) library of intensity/phase signatures, a determination canbe made concerning the properties of the immersion medium utilized inconnection with a known grating structure on the surface. Suchsubstantially unique phase/intensity signatures are produced by lightreflected from and/or refracted by different surfaces and/or immersionmediums due, at least in part, to the complex index of refraction of thesurface onto which the light is directed. The complex index ofrefraction (N) can be computed by examining the index of refraction (n)of the surface and an extinction coefficient (k). One such computationof the complex index of refraction can be described by the equation:N=n−jkwhere j is an imaginary number.

The signal (signature) library can be constructed from observedintensity/phase signatures and/or signatures generated by modeling andsimulation. By way of illustration, when exposed to a first incidentlight of known intensity, wavelength and phase, a first feature on awafer or of an immersion medium can generate a first phase/intensitysignature. Similarly, when exposed to the first incident light of knownintensity, wavelength and phase, a second feature on a wafer or of animmersion medium can generate a second phase/intensity signature. Forexample, a line of a first width may generate a first signature while aline of a second width may generate a second signature. Observedsignatures can be combined with simulated and modeled signatures to formthe signal (signature) library. Simulation and modeling can be employedto produce signatures against which measured phase/intensity signaturescan be matched. In one exemplary aspect of the present invention,simulation, modeling and observed signatures are stored in a signal(signature) library containing over three hundred thousandphase/intensity signatures. Thus, when the phase/intensity signals arereceived from scatterometry detecting components, the phase/intensitysignals can be pattern matched, for example, to the library of signalsto determine whether the signals correspond to a stored signature.

To illustrate the principles described above, reference is now made toFIGS. 12 through 17. Referring initially to FIG. 12, an incident light1202 is directed at a surface 1200, upon which one or more features 1206may exist. In FIG. 12 the incident light 1202 is reflected as reflectedlight 1204. The incident light 1202 and/or reflected light 1204 canpropagate through an immersion medium (not shown). The properties of thesurface 1200, including but not limited to, thickness, uniformity,planarity, chemical composition and the presence of features, can affectthe reflected light 1204. Furthermore, properties of the immersionmedium can include, for example, refractive index and lithographicconstant. In FIG. 12, the features 1206 are raised upon the surface1200. The phase and intensity of the reflected light 1204 can bemeasured and plotted, as shown, for example, in FIG. 17. The phase ofthe reflected light 1204 can be plotted, as can the intensity of thereflected light 1204. Such plots can be employed in connection withknown grating structures built upon the surface to compare measuredsignals with signatures stored in a signature library using techniqueslike pattern matching, for example.

Referring now to FIG. 13, an incident light 1312 is directed onto asurface 1310 upon which one or more depressions 1316 appear. Theincident light 1312 is reflected as reflected light 1314. The incidentlight 1312 and/or reflected light can propagate through an immersionmedium (not shown). Like the one or more features 1206 (FIG. 12) mayaffect an incident beam, so too may the one or more depressions 1216affect an incident beam. Thus, it is to be appreciated thatscatterometry can be employed to measure features appearing on asurface, features appearing in a surface, and properties of a surfaceitself, regardless of features. Additionally, scatterometry can beutilized to measure features of the immersion medium, which can bederived by employing a surface with a known grating structure.

Turning now to FIG. 14, complex reflections and refractions of anincident light 1440 are illustrated. The reflection and refraction ofthe incident light 1440 can be affected by factors including, but notlimited to, the presence of one or more features 1428, the compositionof the substrate 1420 upon which the features 1428 reside andcharacteristics of an immersion medium. For example, properties of thesubstrate 1420 including, but not limited to the thickness of a layer1422, the chemical composition of the layer 1422, the opacity and/orreflectivity of the layer 1422, the thickness of a layer 1424, thechemical composition of the layer 1424, the opacity and/or reflectivityof the layer 1424, the thickness of a layer 1426, the chemicalcomposition of the layer 1426, and the opacity and/or reflectivity ofthe layer 1426 can affect the reflection and/or refraction of theincident light 1440. Additionally, properties of the immersion mediumcan include, for example, refractive index, lithographic constant, etc.Thus, a complex reflected and/or refracted light 1442 may result fromthe incident light 1440 interacting with the features 1428, and/or thelayers 1422, 1424 and 1426. Although three layers 1422, 1424 and 1426are illustrated, it is to be appreciated that a substrate can be formedof a greater or lesser number of such layers. According to one aspect ofthe present invention, a substrate with a known grating structure buildupon it is utilized, which facilitates deriving characteristics of theimmersion medium (e.g., refractive index, lithographic constant) bydetecting scattered light.

Turning now to FIG. 15, one of the properties from FIG. 14 isillustrated in greater detail. The substrate 1520 can be formed of oneor more layers 1522, 1524 and 1526. The phase 1550 of the reflectedand/or refracted light 1542 can depend, at least in part, on thethickness of a layer, for example, the layer 1524. Thus, in FIG. 16, thephase 1650 of a reflected light 1642 differs from the phase 1550 due, atleast in part, to the different thickness of the layer 1624 in FIG. 18from the thickness of the layer 1524 in FIG. 15.

Thus, scatterometry is a technique that can be employed to extractinformation about a surface and/or immersion medium upon which anincident light has been directed. The information can be extracted byanalyzing phase and/or intensity signals of a complex reflected and/ordiffracted light. The intensity and/or the phase of the reflected and/ordiffracted light will change based on properties of the surface and/orimmersion medium upon which the light is directed, resulting insubstantially unique signatures that can be analyzed to determine one ormore properties of the surface and/or immersion medium upon which theincident light was directed.

In order to provide additional context for various aspects of thepresent invention, FIG. 18 and the following discussion are intended toprovide a brief, general description of a suitable computing environment1810 in which the various aspects of the present invention can beimplemented. While the invention has been described above in the generalcontext of computer-executable instructions that may run on one or morecomputers, those skilled in the art will recognize that the inventionalso may be implemented in combination with other program modules and/oras a combination of hardware and software. Generally, program modulesinclude routines, programs, components, data structures, etc. thatperform particular tasks or implement particular abstract data types.Moreover, those skilled in the art will appreciate that the inventivemethods may be practiced with other computer system configurations,including single-processor or multiprocessor computer systems,minicomputers, mainframe computers, as well as personal computers,hand-held computing devices, microprocessor-based or programmableconsumer electronics, and the like, each of which may be operativelycoupled to one or more associated devices. The illustrated aspects ofthe invention may also be practiced in distributed computingenvironments where certain tasks are performed by remote processingdevices that are linked through a communications network. In adistributed computing environment, program modules may be located inboth local and remote memory storage devices.

With reference to FIG. 18, an exemplary environment 1810 forimplementing various aspects of the invention includes a computer 1812,including a processing unit 1814, a system memory 1816, and a system bus1818 that couples various system components including the system memoryto the processing unit 1814. The processing unit 1814 may be any ofvarious commercially available processors. Dual microprocessors andother multi-processor architectures also can be used as the processingunit 1814.

The system bus 1818 can be any of several types of bus structureincluding a memory bus or memory controller, a peripheral bus, and alocal bus using any of a variety of conventional bus architectures suchas PCI, VESA, Microchannel, ISA, and EISA, to name a few. The systemmemory 1816 includes read only memory (ROM) 1820 and random accessmemory (RAM) 1822. A basic input/output system (BIOS), containing thebasic routines that help to transfer information between elements withinthe computer 1812, such as during start-up, is stored in ROM 1820.

The computer 1812 further includes a hard disk drive 1824, a magneticdisk drive 1826 to read from or write to, for example, a removable disk1828, and an optical disk drive 1830 for reading, for example, from aCD-ROM disk 1832 or to read from or write to other optical media. Thehard disk drive 1824, magnetic disk drive 1826, and optical disk drive1830 are connected to the system bus 1818 by a hard disk drive interface1834, a magnetic disk drive interface 1836, and an optical driveinterface 1838, respectively. The drives and their associatedcomputer-readable media provide nonvolatile storage of data, datastructures, computer-executable instructions, etc. for the computer1812, including for the storage of broadcast programming in a suitabledigital format. Although the description of computer-readable mediaabove refers to a hard disk, a removable magnetic disk and a CD, itshould be appreciated by those skilled in the art that other types ofmedia which are readable by a computer, such as magnetic cassettes,flash memory cards, digital video disks, Bernoulli cartridges, and thelike, may also be used in the exemplary operating environment, andfurther that any such media may contain computer-executable instructionsfor performing the methods of the present invention.

A number of program modules may be stored in the drives and RAM 1822,including an operating system 1840, one or more application programs1842, other program modules 1844, and program data 1846. The operatingsystem 1840 in the illustrated computer is, for example, the “Microsoft®Windows® NT” operating system, although it is to be appreciated that thepresent invention may be implemented with other operating systems orcombinations of operating systems, such as UNIX®, LINUX®, etc.

A user may enter commands and information into the computer 1812 througha keyboard 1848 and a pointing device, such as a mouse 1850. Other inputdevices (not shown) may include a microphone, an IR remote control, ajoystick, a game pad, a satellite dish, a scanner, or the like. Theseand other input devices are often connected to the processing unit 1814through a serial port interface 1852 that is coupled to the system bus1818, but may be connected by other interfaces, such as a parallel port,a game port, a universal serial bus (“USB”), an IR interface, etc. Amonitor 1854 or other type of display device is also connected to thesystem bus 1818 via an interface, such as a video adapter 1856. Inaddition to the monitor, a computer typically includes other peripheraloutput devices (not shown), such as speakers, printers etc.

The computer 1812 may operate in a networked environment using logicalconnections to one or more remote computers, such as a remotecomputer(s) 1858. The remote computer(s) 1858 may be a workstation, aserver computer, a router, a personal computer, microprocessor basedentertainment appliance (e.g., a WEBTV® client system), a peer device orother common network node, and typically includes many or all of theelements described relative to the computer 1812, although, for purposesof brevity, only a memory storage device 1860 is illustrated. Thelogical connections depicted include a local area network (LAN) 1862 anda wide area network (WAN) 1864. Such networking environments arecommonplace in offices, enterprise-wide computer networks, intranets andthe Internet.

When used in a LAN networking environment, the computer 1812 isconnected to the local network 1862 through a network interface oradapter 1866. When used in a WAN networking environment, the computer1812 typically includes a modem 1868, or is connected to acommunications server on the LAN, or has other means for establishingcommunications over the WAN 1864, such as the Internet. The modem 1868,which may be internal or external, is connected to the system bus 1818via the serial port interface 1852 to enable communications, forexample, via POTS. The modem 1868 may also, in an alternativeembodiment, be connected to the network adaptor 1866 to enablecommunications, for example, via DSL or cable. In a networkedenvironment, program modules depicted relative to the computer 1812, orportions thereof, will be stored in the remote memory storage device1860. It may be appreciated that the network connections shown areexemplary and other means of establishing a communications link betweenthe computers may be used.

Described above are preferred aspects of the present invention. It is,of course, not possible to describe every conceivable combination ofcomponents or methodologies for purposes of describing the presentinvention, but one of ordinary skill in the art will recognize that manyfurther combinations and permutations of the present invention arepossible. Accordingly, the present invention is intended to embrace allsuch alterations, modifications and variations that fall within thespirit and scope of the appended claims.

1. A system that evaluates a refractive index of an immersion medium,comprising: a receiver that obtains data related to an immersion medium;an analyzer that utilizes the data to identify a refractive index of theimmersion medium; and a memory component in which the identifiedrefractive index of the immersion medium is stored.
 2. The system ofclaim 1, the receiver obtains a signal that traversed through theimmersion medium.
 3. The system of claim 1, the receiver obtains asignal that is at least one of reflected, refracted, or diffracted. 4.The system of claim 1, the immersion medium is at least one of water,perfluorinated polyether, ozone vapor, and supercritical fluid ingaseous phase.
 5. The system of claim 1, the immersion medium has arefractive index greater than the refractive index of air.
 6. The systemof claim 1, the immersion medium is approximately 100% transparent to anexposure wavelength of a signal employed in connection withphotolithographic processing.
 7. The system of claim 1, the analyzerfurther utilizes the data to identify a lithographic constant value (k)of the immersion medium.
 8. The system of claim 1, further comprising asemiconductor wafer that is fabricated at least in part via theimmersion medium.
 9. The system of claim 8, the semiconductor waferincludes a known grating structure.
 10. The system of claim 9, the knowngrating structure is at least one of a series of raised parallel marks,a series of parallel troughs, and a complex grating pattern.
 11. Thesystem of claim 8, further comprising a final optical component utilizedfor semiconductor fabrication.
 12. The system of claim 11, a gap betweenthe semiconductor wafer and the final optical component is filled withthe immersion medium.
 13. The system of claim 12, the gap is filledutilizing a drop of the immersion medium.
 14. The system of claim 12,the gap is filled utilizing a pool of the immersion medium.
 15. Thesystem of claim 1, further comprising a component that varies theidentified refractive index of the immersion medium.
 16. The system ofclaim 1, the analyzer continuously identifies the refractive index ofthe immersion medium.
 17. The system of claim 1, further comprising atransmitter that emits a signal into the immersion medium, the signalyields the data obtained via the receiver.
 18. The system of claim 17,the transmitter is at least one of a frequency stabilized laser, a laserdiode, a helium neon gas laser, and a halogen lamp.
 19. A system thatmaintains a refractive index of an immersion medium, comprising: a firstcomponent that tracks a refractive index of an immersion medium by atleast comparing an observed sire to a stored signature; and a secondcomponent that adjusts the refractive index of the immersion medium whenthe refractive index is outside of a predetermined range.
 20. The systemof claim 19, the second component adjusts the refractive index by atleast one of changing a temperature of the immersion medium, varying apressure of the immersion medium, and displacing the volume of immersionmedium with a disparate volume of immersion medium.
 21. The system ofclaim 19, the second component adjusts the refractive index of theentire volume of immersion medium.
 22. The system of claim 19, thesecond component adjusts the refractive index of a portion of the entirevolume of immersion medium.
 23. The system of claim 19, the firstcomponent tacks the refractive index continuously.
 24. The system ofclaim 19, the first component tracks the refractive index in part bycomparing an observed signature to a modeled signature.
 25. A system,comprising: means for obtaining a refractive index and a lithographicconstant of an immersion medium; and means for changing the refractiveindex and the lithographic constant.
 26. The system of claim 25, furthercomprising means for analyzing when at least one of the refractive indexand lithographic constant fails to be within a predetermined range. 27.The system of claim 26, further comprising means for initiating themeans for changing the refractive index and the lithographic constantwhen at least one of the refractive index and lithographic constantfails to be within the predetermined range.
 28. A method for maintaininga refractive index of an immersion medium within a predetermined range,comprising: assessing a refractive index of an immersion medium;determining whether the refractive index is within a predeterminedrange; and altering the refractive index when the refractive index isoutside of the predetermined range, at least in part by clanging apressure of the immersion medium.
 29. The method of claim 28, furthercomprising: ascertaining a lithographic constant of the immersionmedium; identifying whether the lithographic constant of the immersionmedium is outside of a second predetermined range; and varying thelithographic constant when the lithographic constant is outside of thepredetermined range.
 30. The method of claim 28, assessing therefractive index further comprises comparing a measured signature with astored signature.
 31. The method of claim 28, assessing the refractiveindex further comprises comparing a measured signature with a modeledsignature.
 32. The method of claim 28, altering the refractive indexfiber comprises changing a temperature of the immersion medium.
 33. Themethod of claim 28, altering the refractive index further comprisesremoving the immersion medium.
 34. The method of claim 33, furthercomprising adding a disparate volume of immersion medium.
 35. The methodof claim 28, further comprising stopping photolithographic fabricationupon determining the refractive index is outside of the predeterminedrange.